The CT-Methodology

In principle, a 3D-Computertomograph consists of 3 components, the x-ray source, a rotary table on which the sample is mounted and a digital detector. The x-ray source emits an x-ray beam witch spreads cone-shaped. When the x-ray beam penetrates the sample the intensity is attenuated depending on the sample thickness, the density and atomic number of the material. The digital detector converts the x-ray intensity into a digital projection image. To get a 3D-Image of a sample, a full rotation of the sample within the x-ray beam is essential, so the rotary table stops every angle <1° for taking a further projection-image. Depending on the quality of the 3D-Dataset and the sample, 1000-2000 projections are needed to create a 3D-Dataset.

Figure 1: 3D CT principle

To analyse a 3D-Dataset (also called Voxel-dataset), special software is needed. This Software splits the Volume-data into Axial, Frontal and Sagittal slices to display the results on screen, similar to a metallographic preparation. These three slices are orientated to the global coordinate system of the 3D-Dataset. Axial means a slice in X-Y, Frontal in X-Z and Sagittal in Z-Y direction. By different rendering-algorithms, the software can also display a 3D-Image of the measured sample. The different grey values correspond to the material density whereas high density leads to a bright representation and low density to a dark one.

Figure 2: Dependence of the max. resolution on the object diameter for different CT-systems